A water treatment control system for minimizing trihalomethane formation includes chlorine injection equipment having a plurality of chlorine injectors (11d, 12, 13), a fluorescence analyzer 7 provided on the upstream of the chlorine injectors, and a water flowmeter 6. An chlorine injection rate necessary to reduce a trihalomethane formation potential is calculated by a chlorine injection rate calculating apparatus 14a based on a measured value from the fluorescence analyzer 7. An chlorine injection amount from the chlorine injectors is controlled by chlorine injection amount control apparatuses 15a-15c based on the chlorine injection rate and a measured value from the flowing water flowmeter 6.